The Rock Record

To help in the analysis of Earth’s rock, geologists have divided the history of Earth into time units based upon the fossils contained within the rocks. These time units are part of the geologic time scale, a record of Earth’s history from its origin 4.6 billion years ago to the present. Since the naming of the first geologic time period, the Jurassic, in 1797, development of the time scale has continued to the present. The names of the periods do not change, but the years marking the beginning and end of each unit of time are continually being refined.This scale enables scientists from around the world to correlate the geologic events, environmental changes, and the development of life-forms that are preserved in the rock record.

Geologic Time

The oldest division of time is at the bottom of the geologic time scale. Moving upward on the scale, each division is younger, just as the rock layers in the rock record grow generally younger as you move upward. The time scale is divided into units called eons, eras, periods, and epochs. An eon is the longest time unit and is measured in billions of years. An era is the next-longest span of time; it is measured in hundreds of millions to billions of years. Eras are defined by the differences in life-forms found in rocks; the names of eras are based on the relative ages. For example in Greek paleo means old, meso means middle, and ceno means recent. Zoic means ‘of life’ in greek, and thus Mesozoic means middle life. Precrambian time, which makes up approximately 90percent of geologic time, is divided into Archean and Proterozoic Eons. The Proterozoic in the more recent of the two, and the end of it is marked by the first appearance of organisms with hard parts. All life-forms up until then had soft bodies and no shells or skeltens. Some of these resemble organisms that exist today, such as sponges, snails, and worms, while others cannot be accurately assigned to any kknown animal or plant group.

Plants and Animals...